1. Field of the Invention
The present invention relates to a reference voltage generating circuit device used in various kinds of semiconductor integrated circuits.
2. Description of the Related Art
With development of the semiconductor technique, particularly development of microminiaturization working technique, there has recently been an increasing tendency to the microminiaturization in the field of the semiconductor memory. Due to this, in consideration of reliability of the element and power dissipation of the chip, it is desirable that a power source voltage to be applied to the element be lowered.
In consideration of consistency with the system being used at present, it is desirable that a power source voltage Vcc (EXT) applied from the external portion of the chip can be used as it is for a user, who uses various kinds of systems using the semiconductor memories at present, even if the degree of integration of the semiconductor memory is increased. For this reason, there has been developed a technique in which a supply voltage limiting circuit is provided in the internal portion of the chip, thereby making the voltage applied to the element lower than the external supply voltage.
The supply voltage limiting circuit generally comprises a reference voltage generating circuit, which generates voltage serving as a reference voltage of an internal supply voltage of the chip, and an internal supply voltage output circuit, which keeps the internal supply voltage constant based on the output from the reference voltage generating circuit. Conventionally, in the reference voltage generating circuit, there is used a circuit wherein a resistor R and two diodes D1 and D2 are connected in series. According to this reference voltage generating circuit, if the forward voltage drop of the diodes is VF, a constant output of 2VF can be obtained. Also, if necessary, a higher reference voltage can be obtained by increasing the number of diodes.
However, in such a conventional reference voltage generating circuit, there occur problems when higher voltage is applied to the chip to perform a voltage accelerating test. More specifically, as a first problem, in order to perform the voltage accelerating test in the semiconductor memory wherein a supply voltage limiting circuit is incorporated, it is necessary to control the reference voltage generating circuit so as to obtain higher voltage than the chip internal supply voltage Vcc (INT) when the external supply voltage Vcc (EXT) becomes higher than a normal working voltage. In this case, the desirable relationship between the external supply voltage and the internal supply voltage is that the internal supply voltage Vcc (INT) shows a constant value when the external supply voltage Vcc (EXT) becomes higher than a certain value and the internal supply voltage Vcc (INT) increases again at a constant rate when the external supply voltage Vcc (EXT) becomes the maximum value Vcc max of the normal working voltage. However, in the conventional reference voltage generating circuit using diodes, since a constant reference voltage is generated regardless of the external supply voltage, the conventional reference voltage generating circuit cannot be used as the supply voltage limiting circuit explained above.
The second problem of the conventional reference voltage generating circuit is that stability of the characteristic is insufficient. More specifically, the forward voltage drop of the diode differs in the manufacturing process. Particularly, in a case where a plurality of diodes are connected in series, the influence exerted thereby cannot be avoided. Moreover, the forward voltage drop of the diode differs with temperature.
As mentioned above, there were problems in the reference voltage generating circuit used in the conventional integrated circuit when it was used as supply voltage limiting circuit to perform the voltage accelerating test.